Magnetic materials play an important role in modern technology. They are used in permanent magnets and electromagnets often as part of a motor or other mechanical device. Magnetic materials are also used in various memory devices, transformers, inductances, etc. Various new magnetic materials have advanced this technology considerably and are responsible for the development of new devices useful in modern technology.
Typically, these magnetic materials are made up of such elements as iron, nickel, cobalt, rare-earth metals and alloys of these elements (e.g., FeTb, FeCoTb, etc.).
Particularly attractive in modern technology is the development of magnetic materials for various kinds of memory devices. The development of computers and modern technology has resulted in the need for high density, high capacity memory devices of various characteristics and properties. Various magnetic-type devices such as magnetic computer disks have been used for high density memory units because of low cost, easy fabrication, etc.
Optical disks are also used as memory storage units. Optical disks incorporate low-power lasers to access or read the storage surface. Because laser radiation can be directed onto a very small spot on the storage surface, magnetic storage density is very high, as much as 500 megabytes for an ordinary size disk.
The major disadvantage of conventional optical disks is that they cannot be erased and reprogrammed. Although conventional optical disks are useful in a number of applications, disks with very high bit density that are erasable and reprogrammable are extremely desirable.
A variety of disk structures has been proposed for an erasable, reprogrammable disk. Particularly attractive are disks that work on the magneto-optic principal where magnetic states are used to store the information. In this type of device, the information is contained in magnetic states in the disk, usually in the form of a region of uniform magnetization in a magnetic material with Curie temperature well above room temperature. Reading the disk is done optically generally with a laser using the polar Kerr effect. Changing the direction of magnetization is achieved by heating locally the area of interest and using a magnet or electromagnet to produce the desired magnetization. Such devices have been described in a number of references including a paper entitled "Magneto-optic Recording Technology" by Mark H. Kryder, Journal of Applied Physics 57(1), pages 3913-3918 (15 Aug. 1985) and a paper by I. Sander et al. entitled "Digital Magneto-optic Recorder", published in Optical Data Storage, Di Chen, Editor, Proc. SPIE 382, page 240 (1983).
The nature of the magnetic medium determines, to a large extent, the characteristics of the optical disks, such as data storage density, writing speed, etc. Amorphous thin films of rare-earth transition-metal alloys have shown great promise as materials for magneto-optical mass storage, giving high storage densities and reasonable writing speeds. It is known that the magnetic and magneto-optical properties of such alloys are very sensitive to composition variations. Such composition variations may be caused by oxidation, corrosion or by chemical reaction or interaction (e.g., diffusion) with other materials interfacing with the magnetic materials.
A variety of materials have been investigated as the magnetic storage medium for magneto-optic disks. In addition to various transition-metal elements and rare-earth elements are a variety of alloys comprising rare-earth elements. Typical elements are iron, nickel and cobalt from the transition-metal elements, terbium and gadolinium from the rare earth metals and other elements such as bismuth and tin. Particularly attractive are alloys of TbFe and various compositions of TbFeCo typically ranging from Tb.sub.0.30 Fe.sub.0.32 Co.sub.0.38 to Tb.sub.0.24 Fe.sub.0.35 Co.sub.0.41.
In order to prevent corrosion of these magnetic films, they are often covered by various non-magnetic films such as SiO, SiO.sub.2, Si.sub.3 N.sub.4, etc. Although such non-magnetic protective films improved stability greatly, much greater stability and inertness to external conditions are desirable. Various magnetic materials and protective films for such magnetic materials are discussed in a variety of references including an article by P. Bernstein and C. Gueugnon, Aging Phenomena in TbFe Thin Films, Journal of Applied Physics 55(6), pages 1760-1762 (Mar. 15, 1984) and T. C. Anthony et al., Thermal Stability of Magneto-optic Quadrilayers, Journal of Applied Physics 59(1), pages 213-217 (Jan. 1, 1986).
It is highly desirable to have a magnetic material structure which is inexpensive, highly stable over long periods of time and is suitable for various magnetic devices including memories and optical disk memories.